Carburizing bath



Patented Mar. 20, 1951 CARBURIZING BATH Facundo R. Morral, Stamford, Conn., assignor to American Cyanamid Company, New York, N. Y., a corporation of Maine No Drawing. Application September 8, 1948, Serial No. 48,330

4 Claims. 1

The present invention relates to salt mixtures which, when in a molten condition at elevated temperatures, are adapted for case hardening of ferrous metal, a bath made from such salt mixtures, and a process of using the same.

It is an object of this invention to provide a case hardenin bath that will give penetrations of at least 16 mils in one hour at 1600 F. consistently but which, at the same time, will not leave an insoluble sludge on the article after it is removed from the bath, oil quenched, and shaken in hot water. It is a further object of the present invention to provide an easy washin carburizing bath operable over a temperature range of 1300-l750 F. It is a still further object to rcducethe amount of cyanogen compound commonly required in easy washing carburizing baths by a novel combination of silicon and boron carbides. Still other objects will be obvious from the description of the invention given below.

A cyaniding bath containing unactivated sodium cyanide does not coat the work with insoluble sludges, but has the disadvantage that it carburizes slowly, giving maximum case depths of only about 12 mils at the highest temperature at which it can be operated, about 1550 F., and further that the bath nitrides the work as well as carburizes it, the case at the surface analyzing about 0.6% C and about 0.5% N.

The usual liquid carburizing bath provides satisfactory case depths through the activation of calcium and barium salts. Such a bath may give a case depth of from 16 mils in an hour at 1600 F. to 125 mils in 16 hours at 1750 F. A disadvantage of a bath activated With calcium and barium salts is that these salts sludge, forming oxides and carbonates which adhere to the article when it is withdrawn from the bath and which cannot be dissolved in hot water, but which must be removed irom the article by chemical and/or mechanical means.

In the past, it has been attempted to maintain satisfactory case depths in a cyaniding bath while simultaneously attempting to prevent the formation of insoluble coatings on the articles being carburized by substituting for the alkali metal compound activators some other. activator, such as. silicon carbide. It Was found," however, .that such substitution required an increase in the amount of sodium cyanide over the amount generally used in an activated bath, and that it was also necessary to increase. the bath fluidity by adding a small amount of a fluoride, such as sodium fluoride. These expedients, while avoiding one difficulty, created others. The relative- 2 ly high sodium cyanide content caused the bath to fume at the higher temperatures, while sodium fluoride tended to corrode the bath container. Also, the cover on this type of bath was found difficult to maintain, with the result that sodium cy- 'anide was rapidly converted to sodium carbonate,

disturbing the desired composition of the bath. This type of bath had the still further disadvantages that it was operable only at temperatures below 1650 F. and that its carburizing effect was decreased when used in commercial size pots of 100 pounds or more capacity.

It has now been found that an easy washing carburizing bath operable up to 1750 F., giving satisfactory case depths, can be prepared with a relatively low content of sodium cyanide, no sodium fluoride, and no alkaline earth metal compounds. The bath of the present invention is further characterized by its high activity in commercial size pots, its ease of maintaining a good cover, low sodium cyanide loss, and low nitriding. The means by which these advantages are obtained are believed to be the result of the co-activating effect of about 5% silicon carbide and about 0.1% boron carbide on both the sodium cyanide energizer and on the chemical composition of the bath as a whole. The combination of silicon carbide and boron carbide in this ratio appears to prevent the otherwise complete loss of silicon carbide through its customary reaction with sodium carbonate to form sodium silicate, carbon, and other decomposition products. It has been found that all the boron carbide and about of the silicon carbide thus employed are converted to soluble compounds, probably silicates. The unconverted silicon carbide remaining, apparently.

assisted in some way by the minute amount of boron compound, continues to activate the sodium cyanide, but is never present in sufficient quantity to prevent easy washing of the work after an oil quench. The addition of this small amount of boron carbide, about 0.1%, aids materially in maintaining a good carbon cover on the bath, and the cover in turn reduces sodium cyanide losses to a minimum.

The importance of the coaction between silicon carbide and boron carbide is seen in the following. If siliconcarbide is used without the boron carbide, the case depth is reduced and the cover becomes thin with consequent excessive cyanide loss. And if boron carbide is used without silicon carbide, the case depth is similarly reduced, although the cover is better maintained. However, if the two carbides are used in the proportions disclosed, the cover is properly maintained with low cyanide loss, excellent case depths are obtained, and any material carried over with the articles from the bath is easily washed off.

The bath contemplated in the present invention gives case depths from 16 to 26 mils on SAE steel in an hour at 1600-1750 F., and th film of adherent salts after an oil quench i so soluble that it may be removed simply by shaking the work in hot water a few times. Adherent salts from holes in the work /8 in diameter and deep are easily removed by such means.

The case at the surface analyze typically about 1.0% C. and 0.1-0.3% N., showing low nitriding.

When making up the bath, a salt mixture with the following range of ingredients may be used (parts by weight) 1250 parts sodium cyanide or sodium cyanide equivalent of other cyanogen compound.

32 parts sodium chloride or other alkali metal chloride.

11-31 parts sodium carbonate or other alkali metal carbonate.

5-10 parts silicon carbide.

0.1-1.0 part boron carbide.

0.1-0.5 part graphite.

Within the ranges above given, the following composition is preferred (parts by weight):

32 parts sodium cyanide 32 parts sodium chloride 31 parts sodium carbonate 5 parts silicon carbide 0.1-1.0 part boron carbide 0.14.5 part graphite The bath is immediately operable when melted.

When operating within the range 1300-1650" F., a minimum of 7% sodium cyanid should be maintained in the bath. A range of 1'? %0% is preferred. At temperatures in the range of l700-1'750 R, the cyanogen compound content should be reduced to avoid foaming between the electrodes. At this temperature range, therefore, a cyanogen content as low as 8% calculated on a basis of sodium cyanide may be used, with the preferred range being 10%-12%.

Although sodium cyanide is used as the cyanogen compound in the examples here given, and is preferred on account of its low cost and ready availability, any other water-soluble cysalts containing sodium cyanide or other cy-' anogen compound may be added as indicated one or-more times a day, or the addition m-de before the end of the working day so that the bath will be ready for use the following morning.

The starting mixtures specified above may be used as make-up salts, but it is preferred to use amixturecontaining 30%50% sodium cyanide.

In the following examples, which illustrate but do not limit the invention, a bath was made up comprising about 31 parts by weight of sodium carbonate, 32 parts sodium cyanide, 32 parts sodium chloride, 5% silicon carbide, 0.1% boron carbide, and 0.1 part graphite. The following table shows the variation in carburization penetration on fracture using this bath at various temperatures over various carburization periods on test pieces of SAE 10-20 steel.

Table Case depth in mils (on fracture) Example t g 1 hour 2hours 3 hours 4 hours 0.1 to 0.5 part, the parts being by weight.

2. A mixture, adapted upon fusion for the case hardening of ferrous metal articles, which consists essentially of sodium chloride 32 parts, sodium cyanide 32 parts, sodium carbonate 31 parts, silicon carbide 5 parts, boron carbide 0.1 to 1 part, and graphite 0.1 to 0.5 part, the parts being by weight.

3. A molten salt bath, adapted for the case hardening of ferrous metal articles, formed from a mixture consisting essentially of alkali metal chloride 32 parts, cyanogen compound 12 to 32 parts, alkali metal carbonate 11 to 32 parts, silicon carbide 5 parts, boron carbide 0.1 to 1 part, and graphite 0.1 to 0.5 part, the parts being by weight.

4. A molten salt bath, adapted for the case hardening of ferrous metal articles, formed from a mixture consisting essentially of sodium chloride 32 parts, sodium cyanide 32 parts, sodium carbonate 32 parts, silicon carbide 5 parts, boron carbide 0.1 to 1 part, and graphite 0.1 to 0.5 part, the parts being by weight.

FACUNDO R. MORRAL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,174,867 Bellis Oct. 3, 1939 2,188,063 .Solakian Jan. 23, .1940 2,249,581 Solakian 1- July 15, 1941. 

1. A MIXTURE, ADAPTED UPON FUSION FOR THE CASE HARDENING OF FERROUS METAL ARTICLES, WHICH CONSISTS ESSENTIALLY OF ALKALI METAL CHLORIDE 32 PARTS, CYANOGEN COMPOUND 12 TO 50 PARTS, ALKALI METAL CARBONATE 11 TO 32 PARTS, SILICON CARBIDE 5 TO 10 PARTS, BORON CARBIDE 0.1 TO 1 PART, AND GRAPHITE 0.1 TO 0.5 PART, THE PARTS BEING BY WEIGHT. 